The present invention relates to an electrostatic image forming method applicable to various kinds of imaging apparatuses such as a copier, a facsimile apparatus, a printer and an instant camera.
Electrostatic image forming methods heretofore proposed include an electrostatic image transfer method which was developed by L. E. Walkup and the principle of which is disclosed in U.S. Pat. No. 2,825,814. An approach sharing the same principle as Walkup is disclosed in R. M. Schaffert "Electrophotography" and named "TESI Method No. 5". Another similar approach is taught in "SPSE (1974)" by R. L. Jepsen and G. F. Day of Varian Ass.
An image forming apparatus which employs such a prior art electrostatic image transfer method will be briefly described with reference to FIG. 1. As shown in FIG. 1, the apparatus includes a dielectric recording medium 10 made up of a transparent electrode layer 12 and a dielectric layer 14, and a photoconductive element 16 made up of an electrode layer 18 and a photoconductive layer 20. The recording medium 10 and the photoconductive element 16 are held in close contact with each other. While a DC voltage Vo is applied across the electrodes 12 and 18 from a DC power source 22, a light image associated with a transmissive document 24 is projected from the dielectric layer 14 side so as to provide an electrostatic image on the layer 14. Specifically, among the areas which are exposed to the light, those in which the divided voltage between the photoconductive element 16 and the dielectric recording element 10 has exceeded the breakdown voltage in an air gap 26 between the elements 16 and 10 cause discharging to occur therein and, thereby, allow charges to be deposited on the dielectric layer 14, the charges constituting an electrostatic image. That is, the amount of charge deposited on the dielectric layer 14 depends upon the quantity of exposing light; charges develop in the irradiated portions to provide negative electrostatic images.
The above described type of image forming apparatus uses an inversion-development process because the negative image has to be changed to a positive image. Assuming that the recording medium 10 comprises an electrostatic recording paper, for example, it involves substantial microscopic irregularity due to the configuration of the dielectric layer 14 and the electrostatic characteristics. As reported in various fields in the past, such microscopic irregularity turns an external electric field constituted by the electrostatic image into a minute electric field which is generally referred to as a "microfield", thereby rendering the inversion development considerably difficult.
The prior art electrostatic image forming method as discussed above cannot provide positive images unless the document itself or the light images to be projected are negative. Hence, it is incapable of freely manipulating ordinary documents or the like in forming images.